A novel group of oil-soluble, acidic benzoxazoles has utility as metallurgical extractants. These benzoxazoles are defined by the formula ##STR3## wherein one of R.sup.3 and R.sup.4 is --NO.sub.2 and the other is --H, --NO.sub.2 or an alkyl group of 1 to 20 carbons; and R.sup.1 and R.sup.2 are --H or an alkyl group of 1 to 20 carbons independently, but with the proviso that R.sup.1, R.sup.2, R.sup.3 and R.sup.4 contain a total of at least 6 carbons.
The hydrocarbyl substituents on the benzoxazole molecule must contain a total of 6 to provide the requisite solubility of the metal complexes in extraction solvents such as kerosene. The presence of at least one nitro group, ortho or para to the phenolic hydroxyl, makes the benzoxazole sufficiently acidic to be useful for extraction of metal ions from unneutralized ore-leaches, i.e., leaches having pH's of about 2 or less. The presence of the nitro group (s) also appears to be responsible for an enhanced selectivity for copper over iron.
Several methods of making benzoxazoles are known. For example, U.S. Pat. No. 4,020,081 discloses metallurgical extractants consisting of benzoxazoles which are similar to those of the above formula but contain no nitro groups. They are made by condensation of salicylamides with aminophenols at temperatures of about 200.degree.-240.degree. C.
Benzoxazoles which do contain nitro groups have been made by oxidative ring closure of corresponding Schiff bases, according to two papers by F. F. Stephens and J. D. Bower: The Preparation of Benzimidazoles and Benzoxazoles From Schiff Bases; Parts I & II, J. Chem. Soc., 2971 (1949) and 1722 (1950). They reported obtaining 55-90% yields of the following benzoxazoles by oxidation of the corresponding Schiff bases with Pb (OAc).sub.4 : 2-(p-nitrophenyl)-, 2-(m-nitrophenyl)-, 5-nitro-2-(p-nitrophenyl)-, 5-cyano-2-(p-nitrophenyl)-, 5-carbomethoxy-2-(2-hydroxy-4-nitrophenyl)- and 5-methyl-2-(p-nitrophenyl)-benzoxazole. The authors also reported recovering 2-(p-nitrophenyl)-benzoxazole in amounts which are equivalent to yields of about 72% and 50%, respectively, when chloranil and benzoyl peroxide were employed as oxidants instead of Pb(OAc).sub.4 - which gave an 80% yield of the same benzoxazole. Other oxidants reported by Stephens and Bower as inactive or less effective for the preparation of 2-(p-nitrophenyl) benzoxazole are mercuric acetate, cupric acetate, sodium bismuthate, benzoquinone, N-bromosuccinimide, sulfuryl chloride and hydrogen peroxide/potassium ferricyanide.
Lead tetracetate was tested as an oxidant for preparation of the subject (nitro-substituted) benzoxazoles by the preceding method but was generally found to give multi-component reaction products and/or poor yields of the desired compounds. Similarly unsatisfactory results were obtained with both sulfuryl chloride and hydrogen peroxide. When benzoyl peroxide was used as the oxidant, the reaction was rapid but the results were highly variable, according to the specific Schiff base involved. Although good yields were obtained with the latter oxidant in a couple of instances, the results were generally not good enough to make the use of benzoyl peroxide -- a relatively expensive oxidant -- attractive for use in commercial manufacture of the subject benzoxazoles.
A need for a more efficient and economical method of preparing the above-described nitro-substituted benzoxazoles is thus apparent.